This year we have continued our studies of the molecular basis of drug-induced liver disease, a rare but often life-threatening toxicity that is a major reason for clinical trials of drugs being stopped and drugs being withdrawn from clinical use post-marketing. We have hypothesized that the idiosyncratic nature of this disease is due in part to a deficiency in one or more hepatoprotective factors. In this regard, we have now determined whether interleukin (IL)-6 may also be one of these factors. Following the induction of liver injury with acetaminophen (APAP), a time-dependent increase in liver mRNA expression of IL-6 and its family members IL-11, leukemia inhibitory factor and oncostatin M was observed in wild type (WT) mice suggesting a possible hepatoprotective role played by this cytokine family. Indeed, mice lacking IL-6 (IL-6-/-) were more susceptible than were WT mice to APAP-induced liver injury. The increased susceptibility of the IL-6-/- mice was associated with a deficiency in the expression of hepatic heat shock protein (HSP)25, 32, and 40 as well as inducible HSP70 following APAP treatment. These results suggest that IL-6 and possibly other family members may protect the liver from injury, at least in part, by up-regulating the hepatic expression of several cytoprotective HSPs. In other studies, we have determined whether Kupffer cells, resident liver macrophages, are a source of potential hepatoprotective factors. This was done with the use of liposome-entrapped clodronate (liposome/clodronate), an effective Kupffer cell-depleting agent. It was found that the intravenous injection of liposome/clodronate caused nearly complete elimination of Kupffer cells from the liver and significantly increased susceptibility to APAP-induced liver injury as compared with mice pretreated with empty liposomes. This increased susceptibility was apparently unrelated to the metabolism of APAP since liposome/clodronate pretreatment did not alter APAP-protein adduct levels. Instead, Kupffer cell depletion by liposome/clodronate led to significant decreases in the levels of hepatic mRNA expression of several hepato-regulatory cytokines and mediators, including IL-6, IL-10, IL-18 binding protein and complement 1q, suggesting that Kupffer cells are a significant source for production of these mediators in the liver. Our findings indicate that, in addition to their pro-toxicant activities, Kupffer cells can also have an important protective function in the liver through the production of a variety of modulatory factors, which may counteract inflammatory responses and/or stimulate liver regeneration.